The present invention relates to improvements in so-called ratchet type claw clutches or safety clutches. More particularly, the invention relates to improvements in clutches of the type wherein the claws of a rotary driving clutch element normally mesh with the claws of a rotary driven clutch element to transmit torque to one or more components which are rotated by the driven clutch element, and wherein one of the clutch elements is biased axially toward the other clutch element.
It is known to install in a power tool a safety clutch normally transmits torque to a tool spindle or the like and becomes disengaged when the driven clutch element offers excessive resistance to rotation with the driving clutch element. The two clutch elements are mounted and are rotatable on a common shaft and at least one thereof is movable axially against the opposition of biasing means to disengage its claws from the claws of the other clutch element when the resistance which the driven clutch element offers to rotation reaches a preselected value or when the speed of the driven clutch element exceeds the speed of the driving clutch element. The clutch elements may constitute disks or rings and may form part of or may be rigidly connected to pinions, gears or the like. The resistance which the axially movable clutch element must overcome in order to permit a disengagement of the clutch can be regulated by changing the bias of the spring or springs which urge the claws of the clutch elements into mesh with each other.
For example, when a safety clutch of the just described character is installed in a portable power tool wherein a pneumatic or electric motor normally rotates a screwdriver, a drill or another rotary tool, the clutch insures that the operator is not injured when the tool ceases to rotate or the rotational speed of the tool decreases because the working end of the tool encounters excessive resistance to penetration into or rotation of a workpiece or the like. As a rule, the claws of the clutch elements in ratchet type safety clutches have abutting inclined surfaces which transmit torque under normal circumstances but slide over each other when the tool jams or its rotational speed decreases. Once the clutch is disengaged, the claws of the driving clutch element ride over the claws of the driven clutch element, thereupon enter the spaces between the claws of the driven clutch element, again ride over the claws of the driven clutch element, and so on until the resistance of the driven clutch element decreases or the motor which drives the driving clutch element is arrested. The magnitude of the so-called disengaging moment (at which the driving clutch element becomes disengaged from the driven clutch element) is relatively high, depending on the bias of the clutch spring(s), and the magnitude of the moment which remains and tends to reengage the clutch while the driving clutch element rotates relative to the driven clutch element is also relatively high. In most instances, the remaining or reengaging moment equals or approximates 90 percent of the disengaging moment. This causes excessive wear on the elements of the clutch and compels the operator to exert a very substantial effort in order to hold the housing of the power tool against rotation. The remaining or reengaging moment (while the clutch is disengaged and while the driving clutch element continues to rotate) is less than the disengaging moment because friction between the abutting surfaces of clutch elements is less when the driving clutch element rotates with respect to the driven clutch element. This is due to the fact that the force with which the material of the driving clutch element tends to adhere to the material of the driven clutch element in engaged condition of the clutch (factor .mu.) is higher than when the claws of the driving clutch element ratchet over the claws of the driven clutch element.